Two Problems with Abundant Natural Gas
One problem: It makes greenhouse-effect worries more plausible. As long as fossil fuels looked scarce, it looked like there simply wasn't enough of them to create a major greenhouse problem. Now, it looks like there is enough.
Another problem: The formerly-absurd environmentalist worry of running out of oxygen might not be absurd after all. As long as it looked like fossil fuels could only have been created by photosynthetic life millions of years ago, any fossil fuel would be matched by the oxygen in the atmosphere (and some of that fossil fuel would be too deep to be reachable). In other words, we would run out of fuel before we ran out of oxygen. If Thomas Gold was right and there's far more fuel than was created by photosynthesis, we might run out of oxygen first.